This invention relates to a device used with a conveyer weigher for correcting a dynamic weight signal indicative of the weight of an article provided from the weigher while it is loaded with the article. The invention is used to approximate the weight of the article in its stationary state by using the dynamic weight signal.
As well known in the art, the conveyer weigher is a weighing device having a moving belt or chain conveyer for weighing articles successively fed onto this conveyer under dynamic conditions. This weigher is used, for example, in a weight sorting machine.
As shown typically and illustratively in FIG. 1, the level of weight signal from the weigher rises along curve A when an article to be weighed is fed at time T.sub.0 to the loading end of the weighing conveyer of such weigher. While this level approaches gradually to a specific level W.sub.0 indicative of the static weight of the article, it is a general practice to pick up or sense the weight signal at time t.sub.1 before it arrives at the level W.sub.0 and to unload the article a short time after at time t.sub.2. Accordingly, there is an error d.sub.1, as shown, to be corrected with respect to the static weight W.sub.0.
After the unloading time t.sub.2, the weight signal level decays along curve B and returns to zero at time t.sub.3. It is also a general practice to load the weighing conveyer with a second article at time t.sub.0 ' which is just after the unloading time t.sub.2. The weight signal is picked up or sensed at time t.sub.1 ', well before time t.sub.3, in order to improve weighing efficiency by reducing the article feeding interval Ta. Accordingly, the weight signal of the second article is influenced by the decaying weight signal B of the first article which adds another error d.sub.2, as shown, when it is picked up at time t.sub.1 ' and thus the deviation of the dynamic level from the static level W.sub.0 is d.sub.2 -d.sub.1.
Some devices for correcting this deviation have been proposed already as disclosed, for example, in the opened Japanese patent specification No. 56-168867 and U.S. Pat. No. 4,413,739. In these devices, a test article is weighed first on the weighing conveyer in its stationary state and the test article is repeatedly weighed on the weighing conveyer in its moving state and the resultant dynamic weight signals are averaged. The average deviation of the dynamic weight signal from the static weight signal is calculated and stored in memory and, in the actual weighing operation, the content of the memory is added to or subtracted from the dynamic weight signal of each article weighed by the weigher for cancelling the above deviation. Such prior art correction systems are based upon a principle that there should be very little error if the weight of the test article is close to that of the actual article to be weighed.
As understood from FIG. 1, the error d.sub.2 varies with time t.sub.1 ', that is, with variation of the feeding interval Ta. The prior art correction devices cannot function satisfactorily when the feeding interval Ta varies as, for example, where one or more articles have dropped out from a regular interval line of articles, which may frequently occur in a general production line; the prior art is satisfactory if Ta is maintained constant.
Accordingly, an object of this invention is to provide an improved device which can correct any error induced in the dynamic weight signals, not only when the feeding interval Ta is kept constant, but also when it is subject to changes.